middle 2 mains lost

[BMFB]

so my motors apart again after roughly 3k miles. Yesterday my oil pressure suddenly dropped to about 50% of what it was. It wasn't making much noise, but I tore the motor out last night anyway and took it to my builder this morning. He found that the 2 center main bearings were basically gone (not past the bearing yet but still gone) but that they hadn't spun.

The rod bearings were fine, and the front and back bearings had some scaring, but nothing too bad. The crank was a scat crank that had been turned, and the block had been line honed and balanced before being installed. Bottom end is a 2 bolt with ARP bolts. I asked if it was possible for me to have bent anything and his thoughts lined up with it just blowing apart if that had happened. I wasn't doing anything too crazy, boost was always pretty low (found out why during the tear-down, ****ing exhaust gaskets I mistakenly used and bolts backing off when they crushed). anyway...

any ideas as to how I accomplished this?

 

[TYTILIDIE]

Re: middle 2 mains lost

What kind of exhaust gaskets? Generally we don't use them at all. I wouldn't touch them unless they were copper and you were running headers. What were you running for fuel?

 

[MikeRenz]

Re: middle 2 mains lost

details on the build / block prep?

 

[qbnkiller]

Re: middle 2 mains lost

pm sent

 

[BMFB]

Re: middle 2 mains lost

responded to pm,

block prep was something like tanked, freeze plugs removed/oil gallys cleaned ect., bored 30 over, line honed, milled the deck/heads. I believe that it, I don't know what else there really is,

build was something along the lines of eagle H beam rods, scat crank (.010/.010), phrobe forged pistons, clevit bearings, comp 412 cam, some comp lifters I can't recall PN, lunati springs and I believe long slot rockers, moroso pump stock pressure. l35 heads, 20g turbo... arp head bolts and mains. I think that's everything.

exhaust gaskets were those fluffy metal ones, i don't know what the hell I was thinking. :shoot:

 

[wildphil]

Re: middle 2 mains lost

I think that I used those gaskets on my truck. Didn't know there could be an issue.

 

[ItsMyTy]

Re: middle 2 mains lost

Since your rod and thrust bearing looked ok, that tells me there was no abrasive contamination in the oil. I would suspect the machine work on the crankshaft wasn't done properly.

IMO get a brand new crank or a used one that doesn't need cutting.

 

[BMFB]

Re: middle 2 mains lost

eh my exhaust bolts backed out on me a bit after a while it seems. I noticed it alot more when I was tearing the block down...

as for the crank being machined wrong, do you think that it could have made it 4k miles before letting go with an improperly machined crank? Oil pressure was spot on until right befor eI decided to pull the motor, and there was no sound up until that point.

 

[Don W.]

Re: middle 2 mains lost

any ideas as to how I accomplished this?

Knock. I doubt the crank would have went that far if it was machined wrong or there was some other problem. The 2 center mains have the greatest load.

BTW I run MLS exhaust gasket with no issues. I've even reused them.

 

[SyTyAddict]

Re: middle 2 mains lost

Knock. I doubt the crank would have went that far if it was machined wrong or there was some other problem. The 2 center mains have the greatest load.

BTW I run MLS exhaust gasket with no issues. I've even reused them.

Do the newer 4.3 use mls exhaust manifold gaskets to similar to the LS motors? I've been meaning to check on that, I know they never leak and are reusable. I been thinking that next time I have the manifolds off, that would be my gasket of choice.

 

[BMFB]

Re: middle 2 mains lost

These weren't MLS gaskets, they were 'fluffy' metal crush gaskets.

If it was knock that was hard enough to take out the mains, wouldn't there have been some sign of knock on the top rings or the piston face themselves? everything came out clean...this was a low (18psi) boost high octain motor that I kept a close eye on, it never really saw knock...and there is no sign of pinging on the rings or pistons or plugs or heads...

 

[Quickstop [UK]]

Re: middle 2 mains lost

Crank not machined properly or made of cheap metal that warps under load.

I have the MLS gaskets on the exhausts, not the crush ones you refer to (pics?) and they seem pretty good.

Sorry about this but at least you found it before worse happened.

 

[Try Me]

Re: middle 2 mains lost

Sounds similar to what I found in my motor but i had 3 years and over 40k of abuse on it. We figured the crank flexs slightly under load and that eventually wore the center bearings more than the rest. The marking you saw on the other bearings was probably just material from the centers passing through.

Sounds like the bearing clearance was way too tight on the center mains, or the crank is bent, or the crank isnt machined accurately. Do all of the main journals measure the same? Do all of the main bores in the crank measure the same? Its always possible someone put the wrong bearings in the center (boxed wrong mixed up etc).

 

[fivetodrive]

Re: middle 2 mains lost

Anyone have that old posting showing the problem with the 4.3 mains caps being weak, and that is why ours were upgraded to nodular hense the N stamp on the caps.

 

[Throws]

Re: middle 2 mains lost

Anyone have that old posting showing the problem with the 4.3 mains caps being weak, and that is why ours were upgraded to nodular hense the N stamp on the caps.

If one was building up a new motor... would 4 bolt main caps or GN-style billet caps be a good upgrade compared to the stock components?

-P

 

[BMFB]

Re: middle 2 mains lost

would a 4 bolt get rid of the issue I am having if it was a the 'weak caps' theory?

 

[Quickstop [UK]]

Re: middle 2 mains lost

It's not the caps that seem particularly weak - it's the bolts holding them in and with your ARPs, it's unlikely they let go. 4 is obviously better than two but a lot of people have suggested it's the crank so maybe caps is a red herring.

You even said that the load was pretty average. Can you inspect the crank at a diffferent shop, not tell them what's wrong and see if it comes out true and aligned? Stock cranks seem to last at about 700hp as well - why the scat?

 

[BMFB]

Re: middle 2 mains lost

I talked to my engine guy today and he seems baffeled. He double checked the pistons and everything and found no significant signs of detonation. The only thing is I asked him about how tight the bolts were, and he said he didnt know, becuase he had taken them off with a pnumatic gun...so that won't ever be known...but he's afraid to put it back together until he is sure why it came apart.

 

[Don W.]

Re: middle 2 mains lost

Well my motor did this a couple years ago and I'm pretty sure it was knock. My pistons showed nothing, they're still in the motor. BTW I run a Scat crank. Prolly close to 100 passes last year. Motor still running. Going at it again this year. I check (cut) the oil filter often and no issues. The 2 center mains cary the most load. IE Rods on both sides. That, IMHO, is why they go first.

Your machinist should be able to bolt up the mains and check if the block has misaligned. Maybe not so easy with the crank depending.

Do you still have the plugs? Might be able to tell from them.

Just thinking out loud...

 

[BMFB]

Re: middle 2 mains lost

I looked at the plugs and the only thing i saw worrisome was one (5) had the tip blued just a touch although i didn't see any signs on knock. When you rebuilt it, what did you do differently, or what w=did you have done?